Dogsbody by Diana Wynne Jones

A powerful being fights for his life within the body of a humble, earthbound puppy. Sirius, immortal Lord of the Dog Star and infamous for his quick temper, cannot believe it when he is falsely accused of murder and banished to Earth. There he is reborn into the body of a puppy and learns that he has the life-span of that creature to recover the missing murder weapon. If he fails, he will die. He is adopted by Kathleen, who has no idea that her beloved Leo' is anything more than an abandonded stray. She is a loving owner, but an unwanted guest in a family who mostly resent her presence. Sirius soon learns that he has enemies amongst the humans as well as amongst the unearthly beings who sentenced him. How on earth can he clear his name without his special powers?

Diana Wynne Jones was born in London on August 16, 1934. In 1953, she began school at St. Anne's College Oxford and attended lectures by J. R. R. Tolkien and C. S. Lewis. After graduation, she created plays for children that were performed at the London Arts Theatre. Her first book was published in 1973. She wrote over 40 books during her lifetime including Dark Lord of Derkholm, Earwig and the Witch, and the Chrestomanci series. She won numerous awards including the Guardian Award for Children's Books in 1977 for Charmed Life, the Boston Globe-Horn Book Award in 1984 for Archer's Goon, the Mythopeic Award in 1999, the Karl Edward Wagner Award in 1999, and the Life Achievement Award from the World Fantasy Organization in 2007. Her book Howl's Moving Castle was adapted into an animated film by director Hayao Miyazaki, and the film was nominated for an Academy Award for Best Animated Feature. She died from lung cancer on March 26, 2011 at the age of 76.